1. Field of the Invention
This invention relates to a downhole drilling mechanism that controls the weight and torque applied to a drill bit during operation. More specifically, this invention relates to a constant bottom contact thruster used in a drill string to advance the drill string in pre-selected increments and, therefore, control the weight applied to the drill bit during operation and thus, prevent stalling of the drilling motor.
2. Background
In order to obtain optimum penetration rates, oil and gas operators desire to control the weight on the drilling bit during drilling operations. If excessive weight is placed on the tool, excessive wear and breakage of the tool can occur or the drilling motor will stall.
For most drilling tools, there is an optimum total weight, or a range between a minimum and maximum weight to have on the bit during downhole drilling. However, during actual operations, changes in working pressures and formations, and working in deeper holes can make it difficult to control the weight applied to the tool. Also, when performing horizontal drilling, the drag friction becomes much greater than the dynamic friction. This is particularly true in horizontal drilling utilizing a downhole motor. When using a downhole motor, the drill string does not rotate, but rather primarily functions to guide the downhole motor. Therefore, during such operations, the drill string merely "slides" inside the borehole. Thus, significant static friction acts against the drill string as additional sections of drill string are added. In order to overcome the static pressure acting against the drill string, the operator must provide excess weight on the drill string. When the drill string overcomes the static friction and begins to move, the excess weight must be quickly removed from the drill string or it will cause excess weight on the drill bit, thus, causing the downhole motor to stall. It is difficult for even the most experienced operator to avoid overloading the tool and, at a minimum, it is a very time consuming and fatiguing task.
Prior art methods of controlling the weight on the drill bit include preloading the tool with a telescoping fixture interconnecting the preload drill pipe and drill string so that a constant load is maintained on the tool. However, with such a system, the operator must be able to calculate the accurate preload required. Bumper subs are also used to control weight on drill bits. A bumper sub can be compared to a conventional hydraulic cylinder, similar to those used on construction equipment. The hydraulic pressure in a sub is generated by drill bit differential pressure which is a function of drilling fluid properties, flowrate and bit nozzle size selection. The hydraulic pressure in the sub causes an extension force, or pump open force, to occur. The extension force causes the bumper sub to extend open and thereby exert pressure on the drill bit.
It would be desirable to develop a drill string having a part which allows the operator to control the weight on the drill bit without binding or stalling the downhole motor.